Law enforcement provided Yahoo in November 2016 with data files that a third party claimed was Yahoo user data. We analyzed this data with the assistance of outside forensic experts and found that it appears to be Yahoo user data. Based on further analysis of this data by the forensic experts, we believe an unauthorized third party, in August 2013, stole data associated with more than one billion user accounts. Yahoo has not been able to identify the intrusion associated with this theft. We believe this incident is likely distinct from the incident we disclosed on September 22, 2016. We are notifying potentially affected users and have taken steps to secure their accounts, including requiring users to change their passwords. Yahoo has also invalidated unencrypted security questions and answers so that they cannot be used to access an account.

Separately, our outside forensic experts have been investigating the creation of forged cookies that could allow an intruder to access users’ accounts without a password. Based on the ongoing investigation, the outside forensic experts have identified user accounts for which they believe forged cookies were taken or used in 2015 or 2016. The company is notifying the affected account holders, and has invalidated the forged cookies. We have connected some of this activity to the same state-sponsored actor believed to be responsible for the data theft we disclosed on September 22, 2016.

We are notifying potentially affected users and posting additional information on our website. Additionally, we are taking steps to secure users’ accounts, including requiring users to change their passwords. Yahoo has also invalidated unencrypted security questions and answers so that they cannot be used to access an account.

Based on the ongoing investigation, the outside forensic experts have identified user accounts for which they believe forged cookies were taken or used in 2015 or 2016. The company is notifying the affected account holders, and has invalidated the forged cookies.

For potentially affected accounts, the stolen user account information may have included names, email addresses, telephone numbers, dates of birth, hashed passwords (using MD5) and, in some cases, encrypted or unencrypted security questions and answers. The investigation indicates that the stolen information did not include passwords in clear text, payment card data, or bank account information. Payment card data and bank account information are not stored in the system the company believes was affected.

Hashing is a one-way mathematical function that converts an original string of data into a seemingly random string of characters. As such, passwords that have been hashed can’t be reversed into the original plain text password. At the time of the August 2013 incident, we used MD5 to hash passwords. We began upgrading our password protection to bcrypt in the summer of 2013. Bcrypt is a password hashing mechanism that incorporates security features, including salting and multiple rounds of computation, to provide advanced protection against password cracking.

Forged cookies could allow an intruder to access users’ accounts without a password. Based on an ongoing Yahoo investigation, we believe an unauthorized third party accessed our proprietary code to learn how to forge cookies. The outside forensic experts have identified user accounts for which they believe forged cookies were taken or used. The company is notifying the affected account holders, and has invalidated the forged cookies.

A cookie is a small piece of information stored on a computer for the purpose of identifying a web browser during interaction on websites. Websites use cookies to remember and recognize details about visitors, such as website preferences. Click here for more information on Yahoo practices regarding cookies and similar technologies.

We believe that the August 2013 incident is likely distinct from the incident we disclosed on September 22, 2016.

We have connected some of the cookie forging activity to the same state-sponsored actor believed to be responsible for the data theft we disclosed on September 22, 2016. Those users targeted by the state-sponsored actor were sent an additional notification like the one found here.

Click here to view the content of our notice to affected users. Please note that the emails from Yahoo about this issue will display the Yahoo icon when viewed through the Yahoo website or Yahoo Mail app. Importantly, the emails do not ask you to click on any links or contain attachments and does not request your personal information. If an email you received about these issues prompts you to click on any links, download an attachment, or asks you for information, the email was not sent by Yahoo and may be an attempt to steal your personal information. Avoid clicking on links or downloading attachments from such suspicious emails.

You can change your Yahoo password or security questions and answers by clicking here. We are requiring potentially affected users to change their passwords, and we have invalidated unencrypted security questions and answers so that they cannot be used to access an account.

Although the affected account information did not include passwords in clear text, payment card data, or bank account information, we encourage you to remain vigilant by reviewing your account statements and monitoring your credit reports. Below is contact information for the three nationwide consumer reporting agencies from which you can obtain a credit report.

To protect yourself from possible identity theft, consider placing a fraud alert on your credit file. You also may wish to place a “security freeze” (also known as a “credit freeze”) on your credit file. A security freeze is designed to prevent potential creditors from accessing your credit file at the consumer reporting agencies without your consent. There may be fees for placing, lifting, and/or removing a security freeze, which generally range from $5-$20 per action. Unlike a fraud alert, you must place a security freeze on your credit file at each consumer reporting agency individually. For more information on security freezes, you may contact the three nationwide consumer reporting agencies or the FTC as described above. As the instructions for establishing a security freeze differ from state to state, please contact the three consumer reporting agencies to find out more information.

The consumer reporting agencies may require proper identification prior to honoring your request. For example, you may be asked to provide:

Your full name with middle initial and generation (such as Jr., Sr., II, III)

Your Social Security number

Your date of birth

Addresses where you have lived over the past five years

A legible copy of a government-issued identification card (such as a state driver’s license or military ID card)

Proof of your current residential address (such as a current utility bill or account statement)

You have the right to obtain a police report and request a security freeze as described above. The consumer reporting agencies may charge you a fee of up to $10 to place a security freeze on your account, and may require that you provide certain personal information (such as your name, Social Security number, date of birth, and address) and proper identification (such as a copy of a government-issued ID card and a bill or statement) prior to honoring your request for a security freeze. There is no charge, however, to place, lift or remove a security freeze if you have been a victim of identity theft and you provide the consumer reporting agencies with a valid police report.

For U.S. residents, you can contact the FTC to learn more about protecting your personal information. The contact information for the FTC is below:

No. The systems from which the data was stolen in August 2013 contained no Tumblr user data at the time of the theft. Additionally, Yahoo has no indication that the forged cookies were used to access Tumblr accounts.

If you need further information or assistance with your account, please visit https://help.yahoo.com, where you will find the latest information and may be able to access direct customer support. DO NOT ENGAGE with any support service other than those provided by Yahoo, particularly support service providers that charge a fee for their service. Yahoo does not charge for support service for its accounts. Please note that Yahoo channels all support through https://help.yahoo.com.